The goal of the present studies will be to investigate the molecular mechanisms of action of thyroxine and triiodothyronine. The tissues to be investigated will include the regulation of TSH releaase by the pituitary and the central nervous system. Studies of the ration of T3 derived from plasma to T3 derived from intracellular T4 to T3 conversion will be evaluated under a number of different physiological situations including hypo-and hpyperthyroidism, fasting and in response to various drugs using the rat as a model. Attempts will also be made to isolate and characterize the enzymes responsible for iodothyronine deiodination in the brain and pituitary and compare them with the characterisitics of the similar enzymes in the liver and kidney.